Polyimides are widely used for surface protection films, interlayer insulation films and planarizing films in semiconductor devices, and so on. Recently, polyimides have been used for, for example, insulation films in organic EL devices, planarizing films in TFT substrates, and so on. In these applications, the substrates used have a very large size as compared to those used in semiconductor devices, and therefore, slit coating is generally employed to apply a resin composition. The slit coating technique uses a slit nozzle for coating, which unlike the conventional spin coating technique, does not require the rotation of the substrate to ensure a decreased consumption of resin compositions and improved safety of the process, resulting in a wide use. In performing the slit coating technique, the coating film discharged from the slit nozzle contains a solvent in large amounts, and therefore, it is commonly followed by vacuum drying to remove the solvent quickly after the coating step and subsequent heat drying using a hot plate or the like.
In the slit coating technique, the film thickness depends on a discharge rate from the slit nozzle and a solid content in a resin composition to be coating liquid. Therefore, it is necessary to increase the discharge rate or increase the solid content in the resin composition for forming a thick film. However, when the discharge rate is excessively large, the liquid level swings during transportation of a substrate, so that film thickness uniformity is deteriorated. On the other hand, a resin composition including a polyimide or a polyimide precursor has the problem that when the solid content in the resin composition is increased, the viscosity becomes excessively high.
The viscosity of the resin composition can be reduced by using a good solvent for the resin or a solvent having a low viscosity in itself. The polyimide and polyimide precursor have a rigid structure, and therefore often have low solubility in various solvents. There have been proposed polyimides having improved solubility in organic solvents (see, for example, Patent Document 1) and polyimide precursors having improved solubility in organic solvents (see, for example, Patent Documents 2 and 3). However, even these resins have insufficient solubility in organic solvents. Further, resin compositions having a viscosity suitable for slit coating cannot be obtained from these resins.